Question

我有一个包含用户信息的用户表和一个用userid映射到用户表的签名表，如下所示

    users
      userid | firstname | lastname
        1   |   John       | P
        2   |   Pete       | C
        3   |   Tim        | D

  signs
     doneBy | CheckedBy | VerifiedBy
       1    |   2       | 3

有没有更好的方法来执行此操作而不是多个连接，如下所示，考虑到我会有6个这样的连接

    select 
      concat(usr1.firstname, ' ', usr1.lastname) as doneby,
      concat(usr2.firstname, ' ', usr2.lastname) as checkedby,
      concat(usr3.firstname, ' ', usr3.lastname) as verifiedby
    from signs sgn
      join users usr1 on ( usr1.userid = sgn.doneBy)  
      join users usr2 on ( usr2.userid = sgn.checkedBy)  
      join users usr3 on ( usr3.userid = sgn.verifiedBy)

Answer 1

您可以使用CASE EXPRESSION进行条件聚合，如下所示：

SELECT MAX(CASE WHEN t.doneBy = t.userId THEN t.full_name END) AS doneBy,
       MAX(CASE WHEN t.CheckedBy = t.userId THEN t.full_name END) AS doneBy,
       MAX(CASE WHEN t.VerifiedBy = t.userId THEN t.full_name END) AS doneBy
FROM (
     SELECT u.userid,concat(u.firstname, ' ', u.lastname) as full_name,u.*
     FROM signs s
     INNER JOIN users u
      ON(s.userid IN(s.doneBy,s.CheckedBy,s.VerifiedBy))) t
GROUP BY t.doneBy,t.checkBy,t.VerifiedBy

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